Given two inputs a and b, let da[t] = abs(a[t] - a[t - 1]) and db[t] = abs(b[t] - b[t - 1]). Output a[t] if da[t] is larger, and output b[t] if db[t] is larger. If da[t] == db[t], use whichever input was used last (assume a for the first sample of output).